Convertors such as continuously adjustable hydrostatic transmissions comprising at least one pump and at least one hydraulic motor in at least one of which the volume throughput can be adjusted, are used in drives of driven machines or vehicles. In a transmission of this kind the maximum conversion range is given by the ratio of the sizes of the pump and the motor and of their speeds. One way of further widening the conversion ratio of the drive system beyond this is to use change-speed gears, preferably ones which can be shifted when the drive is under load (load shift gears).
Complex control means ensures that the discontinuous changes of the speed ratio in the drive which are caused by shifting the change-speed gear are at least partially compensated by an inversely proportional adjustment of the continuously adjustable hydrostatic transmission in order to increase reliability and driving comfort or to protect components from sudden alterations in load (cf. DE-OS 23 07 550, 26 52 976, 34 33 494).
When using the machines or vehicles mentioned in the introduction, such as, in particular, construction vehicles there is not only normal operation under load in which there is a positive transmission of power to the change-speed gear from the driving motor, but also so-called overrun operation in which, after reversal of the transmission, there is negative power transmission, i.e. in which the power is transmitted to the driving motor from the change-speed gear.
In the known arrangements, when shifting down to a low gear stage the output speed of the hydrostatic transmission is increased from the speed at the time in order to match this output speed of the hydrostatic transmission to the synchronization point preset by the gear stage and thus to achieve a smooth or relatively smooth shift. This applies particularly in the case of negative powder transmission, as the speed of the driving motor falls by a certain amount owing to the brief interruption or the reduction of the power transmitted when shifting (motor compression). This does not correspond to the basic purpose of changing the gear stage, which is to slow down the machine or the vehicle.